1. Field of the Invention
The present invention relates to a laser irradiation method and a laser irradiation apparatus for performing the laser irradiation (the laser irradiation apparatus includes a laser oscillator and an optical system to guide a laser beam emitted from the laser oscillator to an object to be irradiated). In addition, the present invention relates to a method for manufacturing a semiconductor device comprising a step of the laser irradiation process. It is noted that the semiconductor device described here includes a general device that can be operated by utilizing the semiconductor characteristic. And thereby an electro-optical device such as a liquid crystal display device, a light-emitting device or the like, and an electronic device having such an electro-optical device as its component are also included in the semiconductor device.
2. Description of the Related Art
In recent years, the technology to form a thin film transistor (hereinafter referred to as TFT) over a substrate makes great progress and application development to an active matrix type semiconductor device has been advanced. Especially TFT with the poly-crystal semiconductor film is superior in field-effect mobility (what is called mobility) to TFT with a conventional amorphous semiconductor film and thereby high-speed operation becomes possible. Therefore, it has been tried to control a pixel by a driver circuit formed on the same substrate as the pixel, which was controlled conventionally by a driver circuit provided outside the substrate.
By the way, a substrate for the semiconductor device is expected to be a glass substrate rather than a single-crystal silicon substrate in terms of its cost. However, a glass substrate is inferior in heat resistance and easy to change in shape when it is heated. Therefore, when TFT with a poly-silicon semiconductor film is formed over a glass substrate, laser annealing is performed to crystallize the semiconductor film in order to prevent the glass substrate from changing in shape due to the heat.
The characteristic of laser annealing is that the processing time can be drastically shortened compared with another annealing method by radiation heating or conductive heating, and that a semiconductor substrate or a semiconductor film on a substrate can be heated selectively and locally so that the substrate will be hardly damaged thermally.
It is noted that the laser annealing method described here includes the technique to recrystallize an amorphous layer or a damaged layer formed over the semiconductor substrate or the semiconductor film, and the technique to crystallize an amorphous semiconductor film formed over the substrate. Moreover, the technique to planarize or modify the surface of the semiconductor substrate or the semiconductor film is also included.
The laser oscillators used for laser annealing are classified broadly into two types, a pulsed laser oscillator and a continuous wave (CW) laser oscillator according to its oscillation system. In recent years, it has been known that in crystallization of the semiconductor film, a crystal grain formed over the semiconductor film becomes larger when using a CW laser oscillator than that when using a pulsed laser oscillator. When the crystal grain formed over the semiconductor film becomes larger, the number of the grain boundaries included in the channel region in TFT formed with this semiconductor film decreases and thereby the mobility becomes higher. As a result, such a semiconductor film can be applied to a high-performance device. For this reason, the CW laser oscillator is beginning to attract attention (US published patent application No. 2002/0031876 A1, for example)
Moreover, in order to enhance the productivity in the process of the laser annealing of the semiconductor or the semiconductor film with the CW laser beam, a method is often employed in which a laser beam emitted from the laser oscillator is shaped into the elliptical laser beam (hereinafter referred to as the elliptical beam), and it is irradiated to the semiconductor or the semiconductor film. The reason why the laser beam is shaped into the elliptical is that the original shape of the laser beam is circular or near-circular.